Where do I start...
Firstly, in my gaming experience (which may have either started with Snafu on the Mattel Aquarius, or a ZX81 version of Star Trek), UC2 - Amongst Thieves is probabaly the single most complete "experience" I have had as a gamer. It is stunning from beginning to end - almost a proper interactive movie. I've not had the chance to play TLOU yet, but I hear that is pretty damn good also.
As to should you buy this if you already have them on PS3? IMHO - I can't really see why you would. I have seen UC2 running on the PS4, and for me, whilst it looks pretty - there is something more gritty and "real looking" to the 30fps PS3 version. The 60fps anmations look "jerky" and unnatural compared to the smoother 30fps version also - that's how it appears to me anyway.
UC4 on PS4 is definately making me want to upgrade, however the amount of existing PS3 games that are being re-done and sold at full cost all "sparkly sparkly" on PS4 is cause for concern.